The invention relates to a battery unit including a storage battery with an integral charge control circuit.
The invention applies more particularly to alkaline battery units intended to be charged at a constant current, for example battery units including one or more nickel-cadmium storage cells.
With this kind of storage battery, progressively reducing the charge current at the end of charging significantly increases the service life of the battery unit. To obtain this kind of charging profile, it is conventional to use an external charger designed for the storage battery concerned, that is to say able to detect an end of charge state in order to initiate a progressive reduction in the charging current that it generates. This kind of charger is therefore dependent on the characteristics of the associated battery unit, which increases its cost of manufacture and reduces its flexibility.
U.S. Pat. No. 5,557,188 discloses a regulator that can evaluate the state of charge of the associated storage battery by storing data such as the current entering the storage battery, the current leaving the storage battery, the temperature and a set of parameters specific to the battery unit. The regulator can also communicate with an external system, in particular so that it can be controlled by the external system, for example to carry out a fast charge or a slow charge. This kind of regulator has a high manufacturing cost, which is viable in some particular applications, such as supplying power to a portable computer, but proves to be unsatisfactory in simpler applications, for example supplying power to a mobile telephone.
The object of the invention is to remedy the aforementioned drawbacks by proposing a charge control circuit that is sufficiently simple to be integrated into a battery unit at low additional cost.
To this end, the invention provides a battery unit including a storage battery with an integral charge control circuit, characterized in that the charge control circuit includes a switch adapted to prevent or allow charging current to flow into the storage battery, said switch being controlled by a measuring system for measuring at least one physical parameter representative of the state of the storage battery in order to command successive opening and closing of the switch to chop said charging current if said measured physical parameter crosses a predetermined threshold.
The battery unit is then able itself to manage the current that flows through the storage battery in order to optimize the charging conditions without requiring a dedicated charger.
In one preferred embodiment of the invention, if said predetermined threshold is crossed, the measuring system commands the switch to chop the charging current with a first duty cycle during a predetermined time interval to homogenize the charge of the storage battery and then with a second duty cycle less than the first duty cycle to establish a trickle current for compensating self-discharge of the storage battery. The battery unit is then in an optimum state of charge as long as it is connected to a charger.
In another particular embodiment of the invention, the measuring system includes a voltage divider bridge and one physical parameter representative of the state of the storage battery is a voltage of the storage battery. The battery unit is then better able to evaluate the state of the storage battery, for more refined detection of an end of charge state.
In a further particular embodiment, the battery unit includes a thermostat between said switch and said storage battery to prevent a charging current from entering the storage battery if the temperature of the storage battery increases and the measuring system fails. The battery unit according to the invention is then made safe in the event of failure of the measuring system that it includes.